Modification of semiconductors such as silicon wafers is often implemented by ion implanters, wherein a surface is uniformly irradiated by a beam of ions or molecules, of a specific species and prescribed energy. Usually, the physical size of the wafer or substrate (e.g. 8 inches or greater) is larger than the cross-section of the irradiating beam which deposits on the wafer as a spot of finite size (e.g. 1".times.2"). Commonly, in high current machines, the required uniform irradiance is achieved by mechanical scanning of the wafer through the beam, either by reciprocal motion of the wafer, or a combination of reciprocal motion and rotation about an axis.
It is distinctly advantageous to have a high scanning velocity at least in one direction for a number of reasons: the irradiance uniformity is more immune to changes in the ion beam flux; a higher wafer throughput is possible at low dose levels; for high dose applications degradation from local surface charging, thermal pulsing, and local particle induced phenomena such as sputtering and radiation damage are greatly reduced.
All forms of mechanical scanning are very limited in speed and have the further disadvantage of generating particulates which degrade the surface structures on the wafer.
In a common variation, a time varying electric field is used to scan the beam back and forth in one direction, while the wafer is reciprocated in another direction or rotated about an axis. In this hybrid type of implanter the beam current and hence rate at which wafers can be processed is severely limited by the space charge forces which act in the region of the time-varying electric deflection fields. These forces cause the ions in the beam to diverge outwards, producing an unmanageably large beam envelope. Such a space charge limitation also occurs in implanters which use time-varying electric fields to scan the beam in two directions.
Space charge blow-up is the rate at which the transverse velocity of a beam increases with distance along the beam axis. This is proportional to a mass normalized beam perveance EQU .xi.=I M.sup.1/2 E.sup.-3/2 (1)
where I is the beam current, M is the ion mass, and E is the ion energy. (The Physics and Technology of Ion Sources, Ed. Ian G. Brown, John Wiley & Sons, New York 1989). For typical ion beam configurations encountered in ion beam implanters, space charge effects become limiting at a perveance of .xi..perspectiveto.0.02 (mA) (amu).sup.1/2 (keV).sup.-3/2. Thus, an 80 keV arsenic beam becomes space charge limited at .perspectiveto.1.7 mA, while a 5 keV beam is space charge limited at just .perspectiveto.0.03 mA. For the many commercial processes requiring beam currents greater than 10 mA, and in some cases very low energies as well, it is not viable to scan with time-varying electric fields.
Time-varying magnetic fields, which are used at high frequencies for scanning electron beams, have been suggested from time to time for the scanning of ion beams in implanters, since the space charge forces in general remain neutralized in a magnetic field. Unfortunately, much greater magnetic field energies are required for the deflection of the heavy ions, such as boron (B.sup.+), oxygen (O.sup.+), phosphorus (P.sup.+), and arsenic (As.sup.+), used in ion implanters. Indeed, for comparable beam energies and deflection angles, the magnetic field energy needs to be 10,000 to 100,000 times as large as that required for electrons. Consequently, the techniques developed for rapid magnetic scanning of electrons cannot be scaled to produce a structure suitable for rapid scanning of heavy ion beams. Hitherto, magnetic scanning techniques used in ion implanters have been limited to frequencies of just a few Hertz (Hz).
There are circumstances beyond ion implantation where it is also desirable to produce structures with large working gaps and large magnetic fields a component of which may vary rapidly with time. The performance of such structures is limited by the difficulty associated with producing accurately defined time-varying magnetic fields.